Virtual MIDI Keyboard (v. 1.9)
Virtual MIDI Keyboard (VMK) is an on-screen 36-key piano roll. VMK can be hooked into a synthesizer for playback via JACK. It's pretty simple; just connect the output of VMK to the input of your synthesizer in the ALSA tab of JACK Control. There are some controls for reverb and chorus, as well as a ton of presets.
The default view is a basic piano roll, but the GUI can be extended to include options for key, velocity/pitch, controls, and a program list.
Virtual MIDI Piano Keyboard (v. 0.3.2)
Virtual MIDI Piano Keyboard, or VMPK from here on out, is one of the best virtual keyboards available for Linux. VMPK has 66 keys (as opposed to the 36 of VMK), though still short of the 72 keys in ZynSubAddFX and Yoshimi.
VMPK also includes a ton of built-in instrument sounds, which most other virtual keyboards rely on outside synths for. The view menu can add or remove functionality, including a musical note overlay onto the keyboard itself. This keyboard also adjusts to the size of the application window, making it as big as your screen or as small as your eyes can handle.
Aeolus (v. 0.8.4)
Aeolus is a synthesized pipe organ emulator. This app has no virtual keyboard, so you'll need a hardware keyboard or a software one like Virtual MIDI Keyboard.
The user interface is split into divisions, each with multiple stops. You need to enable each division in the MIDI window in order for any stops in that division to take effect. Once Aeolus is hooked into the system audio playback and a keyboard is connected, everything is smooth sailing.
Hydrogen (v. 0.9.4)
Hydrogen is a drum synthesizer with a number of drum configuration options available. By default, Hydrogen comes pre-loaded with a drum kit featuring kick, stick, snare jazz, hand clap, snare rock, tom low, closed high hat, tom mid, pedal high hat, tom high, open high hat, cowbell, ride jazz, crash, ride rock, and crash jazz instruments. An alternative kit provides access to kick low, kick high, shaker, conga, cymbal and clave sounds. Users can mix and match these components to create custom drum kits. The mixer window controls levels for each instrument in the drum kit separately.
Most of Hydrogen's funtions are contained in the main application window, with the exception of the mixer. Everything about the UI is clean and logical. The upper part of the screen contains patterns, while the lower part houses a notation area for each instrument in that pattern's drum kit. Despite the level of control that Hydrogen presents, this application is shockingly easy to pick up and use. There are no audio frameworks, setup options, or drivers to fool with; this app just works. Whether you are a drummer or just want to fool around with creating beats on your PC, Hydrogen is a great app for anyone.